Recording and reproducing of sound



' Nov. 2, 1937. c. M. BURRILL 2,097,995

RECORDING AND REPRODUCING OF souND` y Filed May 28, 1936 2 sheets-sheevl' vu/wf AMPA/H52 A was M/CBGP/)OME /A/D/CATOE GAIN: Xd EA 1 Y ,weesp/10N: GA/N AMPM/WEE 5 mam Nov. 2, 1937. c. M. BURRILL RECORDING AND REPRODUCING OF SOUND Filed May 28, 1936 CHA/V/VEL A GHANA/EL E GHANA/EL C CHA NIVEL A Fig: 4f

PHO7UCELL A morocELL a HVOTOCELL C INVENTOR Char e6 M Patented Nov. 2, 1937 UNITED STATES PATENT OFFICE RECORDING AND REPRODUCING OF SOUND tion of Delaware Application May 28,

22 Claims.

This invention relates to the recording and reproducing of sound, for example, by means of phonographic records on motion picture film, and has for its principal object the provision of an improved apparatus and method of operation whereby sound having a very wide volume range may be recorded and reproduced without any distortion caused by overshooting or overloading and Without the introduction of any extraneous noise.

Another object is the provision of an improved record such that, with respect to the portion of the record used for reproduction, the level of the recorded sound is maintained as high as possible without overshooting of the sound track.

Another object is the provision of an improved recording and reproducing apparatus which may be readily assembled from parts common to present-day apparatus of this character, with the addition of a minimum of new and special apparatus.

A further object is the provision of a master record capable of reproducing or re-recording sound having a very wide volume range. In such a re-recording, the wide volume range of the master record may be reduced by manual monitoring or otherwise to correspond to the volume range capabilities of the type of final record desired. As commercial requirements change and improvements in methods are made, subsequent re-recordings may be made from the same master record, producing final records of improved characteristics, without the necessity for again obtaining artists, musicians, etc., for another original recording.

As is well known, the range of amplitudes which can be satisfactorily recorded and reproduced by any sound recording and reproducing system is rather denitely limited. On the one hand, as the amplitude increases, a point will be reached, usually called the overload level, where some part of the system no longer functions in the desired manner. On the other hand, as the amplitude decreases, a point will be reached, usually called the noise level, where the perception of the recorded sound in reproduction will be seriously interfered with by extraneous noises generated in some part of the system and superimposed upon the desired sound.

In the case of variablelarea recording on film, the overload limit is or should be set by the overshooting of the sound track. By this is meant the condition where the variable boundary or boundaries serving to define the variable area forming the sound reGQll attempt to pass with-` 1936, Serial No. 82,374

out the strip or track of definite width assigned for the record. The usual result is that they are prevented from doing so by diaphragms or the equivalent, so that the so-called overshot peaks are cut off squarely or limited by the boundaries of the track, and so produce a distorted record.

Other systems of sound recording have different but no less fundamental overloading limitations. For lateral cut disc phonograph records, a limitation is the cutting over of one groove into an adjacent one, and for vertical cut records, a limit is reached when the groove begins to disappear on the upper peaks. For variable density lm records, the non-linearity of the system for high amplitudes produces a more gradual but none the less real overloading or distortion. Y

In each of these systems, random undesirable and unavoidable variations produce a background of noise in the system which limits the minimum amplitudes which can be usefully recorded. In lm recording the most important variations producing noise are the variations in density in a photographic emulsion usually called granularity. Scratches or dirt on the record also may produce noise. Mechanical irregularities in the record groove produce the noise when a disc record is played.

The range between the overload level and the noise level may be called the volume range capability or simply the volume range of a sound recording and reproducing system. All such systems heretofore known are inadequate in volume range for the high quality recording of the best music. Because of this it has been customary to employ manual monitoring to compress the volume range of the subject matter to be recorded to within the limited volume range of the recording system used. That is, during the recording an operator manually alters the gain of the recording amplifier for this purpose, using his judgment and experience to determine his action,

based on his hearing of the sound and on the reading of a volume indicator.

This method is not at all satisfactory, principally because the monitoring operator is unable to anticipate the coming of sudden changes in volume, with the result that his adjustments in such cases are too late to completely prevent overloading or to minimize noise to the fullest extent. Furthermore, the monitoring adjustments must be made smoothly and judiciously, or the artistic value of the recorded matter will be spoiled, hence the monitoring operator must have artistic ability Wll as recording experience, a combination difficult to obtain. At best, the result is the reproduction of sound of limited volume range, less than that of the original, and so lacking in interest, dramatic or artistic value.

Accordingly, it is the purpose of this invention to provide a system of sound recording and reproducing having greatly increased volume range capability, composed of a, plurality of recording and reproducing channels which operate over different and successive ranges of signal level, and

means for fading or switching from one channel.

to another in reproduction, in order to utilize at all times the particular record channel representing the highest level with respect to background noise among those channels not overloaded or overshot at the time. for manual monitoring during recording is eliminated, and instead is substituted the process of determining when to change from channel to channel in reproduction, which may be accom plished by visual inspection or other manual means, leisurely after recording, without any need for anticipation; or which may be done automatically in reproduction. As a result, the reproduced sound is always maintained at a level so high that the effect ofY background noise is negligible.

The invention is very effectively applied to the variable area method of film recording, because in this case overloading is very easily detected, as overshooting, by a visual inspection of the film record. For this reason such an application will be described hereinafter in considerable detail. However, the invention is obviously not limited to the use of any particular method of sound recording, or any particular method of timing or effecting the changes from channel to channel in reproduction. n

The invention will be better understood from the'following description whenconsidered in connection with the accompanying drawings, and its scope is vindicated by the appended claims.

' Referring to the drawings:

Fig. l is a block diagram of a multiple channel recorder constructed in accordance with the invention, Y

Fig. 2 is a similar diagram of a reproducer,

Figs. 3- to 6 are wiring diagrams of different electrical fading circuits, and

Figs. 7 and 8 illustrate various details of an opticalY fading device.

Symphony orchestra music frequently has a volume range of at least 60 db., and it is desirable to be able to record such music. However, usual recording systems have volume range capabilities of not over db., as most optimistically stated. If we Wish a recording of the highest possible quality and set very stringent specications, the discrepancy is even greater. For example, such a high quality recording may be desired for use in wide volume range re-recording experiments, or to preserve a master record, superior to requirements of present commercial records, for future use when commercial records are greatly improved. In such cases, cost would be a minor consideration compared with the high quality of lthe recording obtained, and so a considerable complexity of equipment would be justified. However, the development of special equipment is time. consuming and likely to be exceedingly expensive. Moreover, special equipment is likely to be somewhat lacking in reliability compared with standard commercial devices, and reliability is extremely important'V for recordings of the high- In this way all necessity est grade Where the cost of procuring the artist is very high.

An important object of the invention is to provide a recording of the highest quality, of sound having a wide Volume range, using only a plurality of standard parts.

Standard commercial equipment for recording sound by the variable area method on 35 mm. motion picture nlm, is used in the application to be described. However, the method is not in any Way limited to such a system, but may be applied to any recording system.

We will require, as our specification of quality, that the background noise, hiss, or scratch be at all times entirely negligiblein the inal result, and that at no time shall the nal reproduced sound involve any distortion caused by overloading. Under these conditions, the commercial equipment may be considered capable of perhaps 25 db.

volume range, surely not much more. We will assume that the music to be recorded has a volume range of db., and therefore we will combine, as follows, three complete recording channels, each good for 25 db., in order to handle this 60 db. This leaves an excess capacity of l5 db. to take care of uncertainties in setting the absolute level of recording.

Figure l shows an arrangement wherein three nlm recorders are driven synchronously, and proper synchronizing indications are provided at the start of recording. This is easily accomplished by well known commercial methods. The gain of amplier B is adjusted to be 25 db. greater than that of amplifier A, and the gain of amplier C in turn 25 dbi. greater than that of ampliner B. Thus, if We assume that the amplifiers are so designed that overloading is always caused rst by overshooting the film sound track (as will always be the case with pro-per design), and if E we assume arbitrarily a Zero level for measuring the power applied to the recorders X db. greater than that of the main volume indicator where X db. is the gain of ampliiier A, we have the following overload levels:

For volume levels from 0 to '75 db., as indicated by the main volume indicator, the input to recorder A ranges from 0 to 75 db., that to recorder B from 25 to 100 db., and that to recorder C from 50 to 125 db. Now suppose that the zero level of the volume indicator has been so chosen relative to the gain X of amplier A that the recorders overshoot the sound track at a level of '75 db. Then, since the volume range of each channel is only 25 db., when the level at a recorder is below 50 db. the noise will be too great.

Hence, for input levels of 0 to 25 db. at the volume indicator, the levels at the recorders are: A=0 to 25 db.; B=25 to 50 db.; and 0:50 to 75 db. The records of recorders A and B, being below 50 db., are noisy, While the record of recorder C, from 50 to 75 db., is neither noisy nor overloaded.

Similarly, for inputs of 25 to 50 db., the levels are: A=25 to 50 db.; B=50 to '75 db.; and 0:75 to 100 db. In this case, the record of recorder A is noisy, being below 50 db., that of the recorder C is overloaded, being above '75 db., and that of recorder B, from 50 to '75 db., neither noisy nor overloaded. Likewise, for inputs of 50 to 75 db., the record of recorder A is from 50 to 75 db. and neither noisy nor overloaded, and those of recorders B and C are overloaded, being from 75 to 100 db. and 100 to- 125 db., respectively.

Thus, for any input level from 0 to 75 db., one and only one record is satisfactory withrespect no gr aphs.

to the rigorous requirements we have set down. We have now, in reproduction, only to select at all times the proper record tc use, and to fade smoothly from one tol the other, as required. In the recording, the main gain control must be set so that the volume range of the actual music to be recorded, falls within the -'75 range at the volume indicator. But with a latitude of adjustment of db., this should be accomplished easily. Since, at times, recorder C may be badly overloaded (and recorder B too, to a lesser extent), some care must be taken to prevent damage to the recording galvanometer by such overload. Usually this will be taken care of by making the maximumpower output of the last amplifier stage insufficient to damage the galvanometer.

In reproduction, the simplest arrangement vfrom a functional point of View is to use three synchronized film phonographs, each with its separate amplifier, the outputs of the amplifiers being combined to drive the loudspeaker. The three film records are inspected and the undesired portions of each one painted or blacked -o-ut, so that only the best record is active at any given time. If this is done carefully, the changeovers will be gradual enough to be unnoticeable. This method of eliminating unwanted portions of sound records has long been in commercial use. By visual inspection, overshooting can be very definitely located, but if this overshooting is for too brief a period to be important, the fact is easily observed and the overshooting ignored. The possibility of thus using deliberate judgment in fixing the change-over points is one of the advantages of the visual or manual method over an automatic method for accomplishing the change-overs. Hence the record used can work right up to overshooting,and so the reproduced noise level will always be low.

`F'ig. 2 is a block diagram of such a repro-ducing apparatus. The individual volume controls associated with each film phonograph are used to equalize the sensitivities of the three film pho- These three film phonographs are driven in synchronism, and the three records started properly synchronized by standard and well known methods. The gains in the amplifiers are adjusted in complementary fashion to the gain adjustments used with the corresponding recording amplifiers, as indicated. Thus the outputs from each of the reproducing amplifiers, before combining, assuming each record to be permitted to play al1 the time, would be at the same level. Hence, fading from one record to another, with one and one only active at one time, can be accomplished by blacking out the unwanted records, without a change in level or any noticeable break in continuity. The outputs o f the three amplifiers are therefore combined to drive the loudspeaker, only one record and hence one amplifier, however, being active at any one time, becausey of the blacking out of the unwanted parts of the records. Then all the fortissimo passages will be supplied by record A, all the pianissimo passages by record C, and the average volumes by record B. Hence amplifier A must have large undistorted output, and all the amplifiers must have noise levels below the film noise level. These requirements are not difficult to obtain, however.

By developing some special equipment, considerable simplification of the system above described may be effected. For example, a special three channel recorder, which will reco-rd simultaneously three sound tracks on the' same film, maybe'provided. This would eliminate the necessity for separate synchronizing means and would save film.

To use with this three channel recorder, a special three channel film phonograph is required. This might have three independent optical systems and photocells, feeding three amplifiers, in which case the reproducing system would be just like that of Fig. 2, except for the combining of the three records on one film.

Instead of selecting the proper record to play at a given time by blacking out the unwanted ones, an electrical selecting and fading arrangement may be provided. For example, the fading circuit illustrated by Fig. 3 may be used.

The three running positions are with the slider at A, B, or C, the movement from one to another being made at a suitable speed, by which one amplifier is gradually cut olf and another turned on. At A, amplifier A is connected full on, and the others disconnected; and correspondingly, at B, amplifier B is connected. The two middle sections of the resistance strip have twice the resistance per section as the outside sections, because they are effectively in parallel. Each resistance section may for simplicity have a linear distribution of resistance. Means for moving the slider to the proper position at the proper time will be discussed a little later.

It is preferable, for the smoothest transition, to gradually disconnect one channel and simultaneously to gradually connect the other at the same rate, so that the instantaneous sum of the two combined inputs is constant. This can be done with the circuit of Fig. 4, which involves the use of two sliding contacts moving together.

Here it is essential that the distribution of resistance in each section be linear in order that one channel may be cut in at the same rate as the other is cut out. The two resistors MR are to prevent interaction between the two channels during the transition, and to prevent grounding of the desired channels by the connection to the other slider. To accomplish this, MR must be largecompared with R, i. e., M must be large compared with unity-say for example, 10. In this arrangement, half the output is lost across one of the resistors MR, but this will usually not be serious; It can be avoided by using an additional vacuum tube, as indicated in Fig. 5, but usually this would not be worth while, for the same tube could be used otherwise to give a gain of more than two.

In the above fading arrangements, it has been assumed that the gain in the three channels has already been adjusted to the proper values, so that the outputs of the channels, neglecting distcrtion, are at the same volume level. also possible to accomplish the necessary fixed differences in gain in the fading arrangement. Thus the fader may be made to follow the photocells directly, so that only a single amplifier would be required. Fig. 6 shows a way of doing this. Here n is the fixed difference in gain between channels in db. and N is the corresponding voltage ratio. In the numerical example We have been using, 1L=25 and N=1'l.8.

Figure 6 shows the currents produced by three photocells of identical characteristics, playing the three records made at the different levels. If the photocells are not identical, they may be equalized by adjustment of their polarizing voltages, by auxiliary voltage dividers, or otherwise. This being done, and assuming the distribution It is` of resistance in each section to be linear; a study.;

of this diagrarn'will 'show that, neglecting: dis,- tortion, the output is independentof the position of the two coupled sliders.

i In .thiscase, theoretically, the fading-could be done-at any convenient speed. Practically, how-y ever, it will still be desirable to do it as rapidly as convenient, provided this, as will likely be the case, is slow compared with a single cycle of the lowestmusic frequency. There is a possibility of trouble. with noisy contacts Yin suchy a fading scheme, because of the low energylevel at which the fading is done. However, this should be no worse than the low level mixing now commercially used with ribbon microphones.

In the above description of fading methods in which one channel is cut in at the same rate as the other is cut out, it has been assumed that the records pertaining to the different channels are exactly' synchronous, so that voltages from the two channels will always add in phase. How; ever, in practice this may not be the case, particularly if separate films are used for each channel.- Theseparate channels may be accu-` rlately enough synchronized as far as the ear is concerned, and yet the voltages from two channels` may not add at constant phase. In this case it will be desirable to use suitably tapered or graded resistance variation instead of linear .resistance variation in the fading circuits of Figs. 4, 5, or 6, to obtain the smoothest transition.

For example, if the voltages from two channels are to add in random phase, at the mid-point of a changeover, per cent. of the output of each :channel should be combined, instead of 50 per cent. for the case of exact synchronisrn.

The fading from one channel to another may be done optically instead of electrically, by means of arrangements closely analagous to the electrical methods just discussed. The light beam corresponding to a particular channel may be attenuated by means of an absorbing wedge or other absorbing means at any point, preferably, however, not between the slit and the lrn, because the more or less unavoidable scattering which would accompany the absorption would interfere with the sharpness of the image of the slit on the film. If the attenuation is placed ahead oi the lm, the effect is to attenuate the light carrier, later to be 4modulated a xed percentage. If the attenuation is placed beyond the lm, the modulated beam is attenuated, the result being the same in each case.

If the required i'lxed diierences in gain between the different channels are obtained electrically in the amplifiers following the photocells, the optical fading from one channel to another is analagous to the electrical diagrams of Figs. 4. and 5, except that in the optical case there is no diiculty with interaction between channels operating simultaneously. To offset this advantage, there is this difficulty-that most light attenuators are logarithmic, rather than linear as is desirable when one channel is cut off and another turned on simultaneously. For this reason, it will probably be desirable to attenuate the light by intercepting completely a variable part of the area of a beam, in such va. way that the intensity of the illumination of the scanning slit is varied without disturbing the uniformity of its illumination, rather than to attenuate the Whole beam by absorption. Of course, the former method would have to be applied before modulation, when the beam is homogeneous.

One way of accomplishing this would be by varying the aperture of the optical system, as

i indicated in Figs. 7 and 8. In this arrangement,

the source lil is imaged at the objective H|2, andthe slit I3 is imaged at the lm lli. A movable shutter l5 is placed in the plane of the optical center of the objective (between the two components Il and i2, usually,) to accomplish the fading attenuation. Vle require then to form a rectangular image i6 of the source, just back of the attenuating shutter I5, andthen to cover it gradually by moving the shutter parallel to the long side oi the rectangle. Then the light flux not intercepted is a linear function of the displacement X of the shutter.` The rectangular image is obtained by using an-approximately rectangular source, such as-the usual long coiled lament incandescent lamp. The photocell is indicated at il'.

The required xed differences in gain between the diierent channels may also be obtained in combination with optical fading, in a manner analagous to the electrical circuit shown in Fig. v6, by adjusting the'maximurn'illumination received by each sound track to the proper value. The difficulty here is to obtain a suicient range of illumination with'practical light sources, without diinculty from stray illumination and resulting noise. owever, with proper care in design, the result can be realized.

If the fading is done rapidly, it may not be necessary to accurately match the rates of cutting in and cutting out of the two channels; hence the reiinement of using linearflight attenuators may not be necessary. Therefore, the usual approximately logarithmic absorbing wedge maybe satisfactory.

So far nothing has been said about how the fading mechanism, whether optical or electrical, is actuated. One method is to use a relay operating on a series of impulses, similar to an automatic switching telephone-selector relay; The actuating electrical impulses may be generated by the illm, automatically, by a variety of ways. For example, a series of holes may be punched in the film, properly spaced-in a part of the lm not used'for sound track. If then the Y over a hole, thus generating the requiredimpulses. One impulse might signify to fade to channel A, two impulses to channel B, etc., and the selector would be connected to energize the proper solenoids for electrically operating the fading mechanism accordingly. Or the power for operating the fading might be taken from the main iilm driving mechanism at the most convenient point. to release catches or engage clutches.

As in the case of fadingfby painting out the unwanted track sections, the desiredy changeover points would be obtained by inspection of the film for overshooting.v Then the changeover sighals, punch marks for example, might be placed on thelilm manually, a proper distance in ad-y van-ce of the point where the change must be complete, to allow for any unavoidable timelag in the mechanism. Since it would be inconvenient in the inspecting process to place the fading signal very far fromvthe overshooting requiring Thus the selector would be usedv it, it will be inconvenient if this time lag exceeds v one second or one and one-half feetY of lm at standard lm Speeds, assuming that in playing the iilm, the fading signal and sound are scanned at approximately the same point. :If

. even in extreme cases, or so as to eliminate noise,

the fading signal is taken from `the film at the proper distance in advance of the sound scanning, then the signal may be placed right at the overload point. In any case, it will be easy enough to provide fading apparatus having a total time lag of less than a second, since the automatic telephone accomplishes a much more complicated switching in only five seconds (dialing and connecting number 858).

In order that the fading signal may be placed on the negatives and then reproduced automatically on all the positive prints in the normal printing process, it may be desirable to use an optical fading signal pick-up instead of the mechanical contact finger, an auxiliary signal sound track as it were. In this case, the signal placed on the negative might loe a dot of black paint instead of a hole.

The recording and reproducing method which we have described above may be defined in its most general aspect as follows:

A plurality of recordings are made, so that at all times at least one record is free from distortion and ground noise. Then, in reproduction, either automatically or in a prearranged schedule, the best of the plurality of records is always used. This is analagous to the well known diversity method of receiving short wave radio signals. In this method, a plurality of receivers are used, connected to independent antennas spaced at least a major portion of a wave length apart. Automatic means are then used to select, at any given time, the receiver giving the best signal and connect it to the subsequent apparatus.

In this case, selective fading is the difficulty to be overcome, and this is chiefly a matter of fading out of the carrier producing the equivalent of over-modulation. Hence the best signal is recognized, with only occasional error, as the one having the strongest carrier.

However, with multiple sound records, the best one to use is the one giving an output just as little as practical less than the overload level of the system. The diii'iculty is in devising an automatic mechanism capable of selecting the best channel on such a basis, which will be fast enough to prevent overload, and which will not play too safe, i. e., work too far below overload most of the time. A practical compromise in the automatic selection of the best record might be to use the one having an average level nearest a pre--r determined fixed value,

Such automatic selecting means might well utilize a bridging amplifier, if needed, and a peak voltmeter on one or more channels. By peak voltmeter I mean one whose readings vary proportionally to the envelope of the music wave; a grid leak detector is a well known example oi such a device. The time constant of such a voltmeter, together with the rapidity of the fading mechanism when actuated, will determine how near to overload the automatic fading mechanism can be set to operate without objectionable overloading occurring. It is likely that such automatic selection could be made to work satisfactorily with average program material, but for extreme cases of very sudden changes in volume it would likely fail to prevent overloading or to eliminate noise, depending on how it was adjusted. By giving the actuating voltmeter and mechanism an asymmetrical characteristic, i. e. one slow to select a higher gain channel and fast in selecting a lower gain channel, or vice versa, one could make sure that the record selection was always made so as to prevent overloading but not necessarily both all the time. For example, if the time constant of the mechanism is made very rapid in following rising voltages, but slow in following falling ones, then the adjustment maybe made so that overloading will always be prevented, but noise will not be complete? ly eliminated rightafter a rapid decrease in volume. Or conversely, if the mechanism is made slow in following a rising volume level and fast in following a'falling one, noise will aways be eliminated, but overloading will follow a sudden increase in voume. Such asymmetrical time constants are easily obtained with suitable rectier circuits. i

I claim as my invention:

1. The sound record producing method which includes recording the sound on a plurality of tracks at different levels, segregating the high level but not overshot sections of said tracks, and combining said sections to produce said record.

2. The background noise eifect minimizing method which includes recording the sound on a plurality of tracks at different levels, segregating the high level but not overshotsections of said tracks, and reproducing only said sections.

3. The background noise effect minimizing method which includes independently recording the sound at .different levels, segregating the high level but not overshot sections of said independent recordings, and fading from one to another of said sections during the reproduction of said recordings. Y 4. The method of Vreproducing sound from a plurality of tracks recorded at different levels which includes reproducing sound from one of said tracks, and fading to another of said tracks when said rst mentioned track is under or over modulated.

5. The method of reproducing sound from a plurality of tracks recorded at different levels which includes reproducing sound from one of said tracks, and electrically fading to another of said tracks when said sound is under or over modulated.

6. The method of reproducing sound from a plurality of tracks recorded at different levels which includes reproducing sound from one of said tracks, and optically fading to another of said tracks when said sound is under or over modulated.

7. The method of increasing the volume range capability of a sound record which includes recording the sound at diiferent successive ranges of level, segregating the best modulated sections of said recordings, and combining said sections to produce said record.

8. The method of minimizing background noise and sound distortion due to overmodulation which includes independently recording the sound at a plurality of different successive levels, segregating from said recordings successive sections which are most free from background noise and distortion due to overmodulation, and combining said sections to form a complete record.

9. The method of minimizing background noise and sound distortion due to overmodulation which includes independently recording the sound at a plurality of different successive levels, segregating from said recordings successive sections which are most free from background noise and distortion due toI overmodulation, and reproducing only said sections.

10. A sound recording and reproducing system including means for recording the sound at a plurality of different levels, means for selecting sections least distorted by background noise and overmodulation, and means for reproducing only said selected sections.

11. A sound recording and reproducing system including means for recording the sound at a plurality'of different levels, means for selecting sections least distorted by background noise and overmodulation, and means for fading from one to another of said sections.

12. A sound recording and reproducing system including'means for recording the sound at a plurality of diierent levels, means for selecting sections least distorted by background noise and overmodulation, and electrical means for fading from one to another of said sections.

13. A sound recording and reproducing system including means for recording the sound at a plurality of different levels, means for selecting sections least distorted by background noise and overmodulation, and optical means for fading from one to another of said sections.

14. A sound reproducer including a plurality of light responsive devices subjected to light beams modulated at diierent levels, a signal channel, and means interposed betweenY said devices and 'said channel for fading from one of said devices to another.

15. A device Afor reproducing sound from a plurality of tracks recorded at diierent levels including an optical system, and means interu posed in the path of said system for fading from one of said paths to another.

16. In a reproducing vsystem employing a plurality of recordings of the same sound at dif-v ferent levels, the combination ofmeansfor selectively reproducing said necordings, means for equalizing the outputs reproduced from said recordings, and means for changing from one to another of said`outputs.

17. In a system adapted to reproduce sound from a plurality of synchronous records of the same sound recorded at different levels, the combination of means including ampliers having such gain ratios as to reproduce said records with substantially the same output, and means for switching from one to another of said amplifiers.

18. In a system adap-ted to reproduce sound from a plurality of synchronous records of the same sound recorded at different levels, the combination of means including ampliers having such gain ratios as to reproduce said records with substantially the same output, and means operable to switch from one to another of said ampliers Without the production of extraneous noise.

19. The background noise effect minimizing method which includes recording the sound on a plurality of tracks at different levels, visually inspecting said tracks to identify their high level but not overshot sections, segregating said sections, and reproducing only said sections.

20. The method of reproducing sound from a plurality of tracks recorded at diierent levels which includes reproducing sound from one of said tracks, and switching to another of said tracks When said first mentioned track is under or over modulated.

21. A sound recorder including a sound pick-up device, la plurality of recording devices, a plurality of audio frequency channels interposed between said pick-up device and said recording devices and each adjusted to` operate at a diierent output level, and means for protecting said recording devices from damage due to overloading.

22. The combination of means for recording sound at a plurality of different levels, means for selecting sections least distorted by background noise and overmodulation, means for fading from one to another of said sections, and means for equalizing the outputs from said sections.

CHARLES M. BURRILL. 

